An axle driving apparatus, which includes a common housing for housing therein a hydrostatic transmission and a pair of axles differentially connected thereto and extending transversely to each other so that an oil sump for storing therein oil for operating the hydrostatic transmission is formed in the housing is well known, as described in U.S. Pat. Nos. 4,914,907, 4,932,209 and 5,040,649.